The invention relates to a hydraulically operated hoisting apparatus for a ship for picking floating objects up out of the sea.
It is known to pick up floating objects from a ship using a crane having a boom which projects over the side of the ship, while a cable connected to a winch and to the object which is to be picked up is suspended from the upwardly inclined boom. When the winch is operated, the cable is drawn up and the object is hoisted out of the water.
With a slow hoisting speed, there is a danger, particularly if the sea is fairly rough, that the movements of the ship which are transmitted to the crane will cause the suspended object to strike the ship or the surface of the water once or repeatedly, which can result in damage to or destruction of the object.
In order to prevent the object or equipment which is being picked up from colliding with the ship or crane, the hoisting speed must be greater than the sum of the velocities resulting from the pitch and roll of the ship, on the one hand, and the orbital speed of the sea swell, on the other hand. In unfavourable cases, all these velocities may be added together and in certain circumstances may reach a speed of several meters per second.
A considerable drive power is necessary in order to attain such a high hoisting speed. Known heavy duty hoisting equipment will not produce the hoisting speeds needed. Moreover, more powerful drives of this kind also require energy supplies greater than those usually available on board ship.
With a crane, there is also the disadvantage that the freely suspended cable may swing sideways over a large part of the hoisting distance. Pendulum movements of this kind may cause impact against the side of the ship and result in damage to or destruction of the object or equipment which is being picked up.